[libvirt] [PATCH] Qemu driver: Support network-backed pflash disks.

[Prerna Saxena]

This implements support for firmware loader & NVRAM disks over network-backed disks.
As discussed in https://www.redhat.com/archives/libvir-list/2018-March/msg01721.html,
the patch embeds the <source> spec for disks in <loader> and <nvram> elements as well.

Currently, the source type is annotated by introducing a new attribute "backing" for both
'loader' and 'nvram' elements. Hence, a sample XML with new annotation looks like this:

<loader readonly='yes' type='pflash' backing='file'>
  <source file='/usr/share/OVMF/OVMF_CODE.fd'/>
</loader>
<nvram backing='network'>  <source protocol='iscsi' name='iqn.2013-07.com.example:iscsi-nopool/0'>
  <host name='example.com' port='6000'/>
</source>

The patche automatically re-formats any older-stype declaration into this new style.
Templates can be used to create a new NVRAM only if the nvram backing = 'file'.